Acetyltransferases play important roles in such diverse processes as circadian rhythm and the regulation of gene expression. Serotonin N-acetyltransferase is responsible for the circadian pattern of melatonin production in the pineal gland of animals and people. Histone acetyltransferases are proposed to modulate the remodeling of chromatin by catalyzing acetylation of key lysine residues on histones. They are also thought to serve as catalysts for the acetylation of a variety of transcription factors. The principal goals of this proposal concern the design, synthesis, and evaluation of inhibitors of serotonin N-acetyltransferase, the melatonin rhythm enzyme, and the histone acetyltransferases (HATs) PCAF and p300. In addition, the principal investigator seeks to better define the key determinants for HAT substrate recognition which may lead to the discovery of new biological substrates. The principal investigator seeks to build on initial findings that bi-substrate analogs can lead to potent and selective inhibitors of these enzymes. He will examine a range of modifications to enhance binding affinity, selectivity, and cell permeability. Designed compounds will be tested in vitro, potent ones will be examined using X-ray crystal structure analysis with the appropriate enzyme where possible, and in vivo in a variety of different systems. HAT substrate selectivity will be evaluated using peptide and protein substrates, using classical approaches as well as combinatorial libraries. Inhibitors of serotonin N-acetyltransferase will be useful as tools to evaluate the role of pineal serotonin and melatonin in circadian physiology and potentially as lead compounds for mood and sleep disorders. Selective inhibitors of HAT enzymes may be useful in the treatment of cancer, HIV, and other diseases related to the dysregulation of gene expression.